The above application (“the '545 application”) describes a method and apparatus for determining acetabular component positioning, particularly for use in connection with hip arthroplasty. The method establishes a coordinate frame for the ipsilateral hip, and an apparatus is disclosed that rapidly and reliably establishes the desired frame. A preferred form of the apparatus is shown in FIG. 4 of the '545 application, and comprises a manual stereotactic instrument in the form of a tripod having of a pair of extensible arms extending from a common hub about which the arms can be rotated. First and second legs or cannulas, respectively, extend from an end of the respective arms remote from the hub and generally perpendicular to the plane formed by the arms, and a third leg or cannula extends from the hub, also generally perpendicular to that plane. The tips of the legs remote from the plane themselves are then positioned by the surgeon or by the instrument itself.
One of the tips is placed at the root of the ischium, a short distance (e.g., 20 millimeters or so) above the infracotyloid notch; this establishes a “basepoint” of Anchor point for proper docking of the instrument to the hip. A second tip is placed by the surgeon on the lateral side of the iliac wing, adjacent to the anterior superior iliac spine. The third tip then lands on the surface of the lateral ilium, anterior to the sciatic notch; the precise location is determined by the settings of the instrument, in particular, the extensions of the arms and the angle between them. The tips of the three legs, when so positioned, themselves form a plane and provide a reference frame with respect to which the orientation of the hip may be referenced.
As described in the '545 application, an alignment guide in the form of a rod (cf. rod 128 of FIG. 4 of the '545 application) is affixed to the instrument and is set at a defined orientation with respect to the instrument; this orientation defines the orientation at which an acetabular cup is to be inserted into the acetabulum by the surgeon. The insertion is accomplished with the aid of a cup inserter, an instrument onto which an acetabular cup is removably fitted at one end and having a handle at the other end by means of which the surgeon can position the cup and apply force to seat the cup when it is maneuvered into the desired orientation in the acetabulum indicated by the alignment guide. At least a portion of the cup inserter, typically the handle, has a straight segment which can be aligned with the alignment guide rod by visual inspection.